Usama bin Zayd reported,

The Messenger of Allah ﷺ sent us on a military expedition to the Huraqat. They became aware of our approach and fled, but we caught up with one man. When we closed in on him, he said, "There is no god but Allah." We struck him down and killed him.Something about this troubled me, so I mentioned it to the Messenger of Allah ﷺ.He said, "Who will stand for you against 'There is no god but Allah' on the Day of Resurrection?"I said, "Messenger of Allah, he only said it out of fear of the weapon and being killed."He said, "Did you split open his heart so you could know whether he said it for that reason or not? Who will stand for you against 'There is no god but Allah' on the Day of Resurrection?"He kept repeating this to me until I wished I had not embraced Islam until that very day.

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أسامة بن زيد قال

بعثنا رسول الله ﷺ سرية إلى الحرقات فنذروا بنا فهربوا فأدركنا رجلا فلما غشيناه قال لا إله إلا الله فضربناه حتى قتلناه فعرض في نفسي من ذلك شيء فذكرته لرسول الله ﷺ فقال من لك بلا إله إلا الله يوم القيامة قال قلت يا رسول الله إنما قالها مخافة السلاح والقتل فقال ألا شققت عن قلبه حتى تعلم من أجل ذلك أم لا من لك بلا إله إلا الله يوم القيامة قال فما زال يقول ذلك حتى وددت أني لم أسلم إلا يومئذ